2/4/ Naming Alkanes. Naming Alkanes. Naming Alkanes

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1 3.4 Systematic nomenclature devised by the International Union of Pure and Applied Chemistry (IUPAC) Name has four parts 1. Prefix which specifies the location of functional groups and other substituents in the molecule 2. Parent selects the main part of the molecule and tells the number of carbon atoms 3. Locant gives location of primary functional group 4. Suffix identifies functional group family it belongs to Alkanes can be named by following four steps 1. Find the parent hydrocarbon a) Find the longest continuing chain of carbon atoms in the molecule b) If two different chains of equal length are present, choose the one with the larger number of branch points as the parent 2. Number the atoms in the parent chain a) Number each carbon atom in the parent chain beginning at end nearer the first branch point b) If there is a branching point at an equal distance from both ends of the parent chain, begin numbering at the end nearer the second branch point 1

2 3. Identify and number the substituents a) Assign a number (called the locant) to each substituent to locate its point of attachment to the parent chain b) If there are two substituents on the same carbon, give them both the same number. There must be as many numbers in the name as there are substituents 4. Write the name as a single word If two or more different substituents are present write them in alphabetical order If two or more identical substituents are present, use the multiplier prefixes di-, tri-, tetra-, and so forth Do not use these prefixes when alphabetizing 5. Name a branched substituent as though it were itself a compound Begin numbering the branched substituent at its point of attachment to the main chain Substituent alphabetized according to the first letter of its complete name, including any numerical prefixes, and is set off in parentheses when naming the entire molecule 2

3 More examples Some alkanes have nonsystematic, common names 1. Three-carbon alkyl group 2. Four-carbon alkyl groups 3. Five-carbon alkyl groups Some compounds can be named with IUPAC rules or with common names 3

4 Alphabetization in the naming of alkanes Nonhyphenated prefix iso- is considered part of the alkyl-group name when alphabetizing Isopropyl and isobutyl are listed alphabetically under i The hyphenated and italicized prefixes secand tert- are not considered part of the alkylgroup name when alphabetizing Sec-butyl and tert-butyl are listed alphabetically under b Worked Example 3.2 Practice in What is the IUPAC name of the following alkane? Worked Example 3.3 Converting a Chemical Name into a Structure Draw the structure of 3-isopropyl-2-methylhexane. 4

5 3.5 Properties of Alkanes Alkanes referred to as paraffins Latin Parum affinis meaning little affinity Show little affinity for other substances Chemically inert to most laboratory reagents React with oxygen, halogens, and a few other substances under the appropriate conditions Reactions of alkanes with oxygen Occur during combustion in an engine or furnace when the alkane is used as a fuel Methane reacts with oxygen CH O 2 CO H 2 O kj/mol (213 kcal/mol) Properties of Alkanes Reactions of alkanes with the halogen Cl 2 Occurs when a mixture of alkanes and Cl 2 is irradiated with ultraviolet light Denoted hn A sequential substitution of the alkane and hydrogen atoms by chlorine occurs Results in a mixture of chlorinated products Methane reacts with Cl 2 Properties of Alkanes Alkanes show regular increases in both boiling point and melting point as molecular weight increases Due to the presence of weak dispersion forces between molecules Dispersion forces increase as molecular size increases Melting and boiling occur when sufficient thermal energy is applied to overcome dispersion forces 5

6 3.6 Stereochemistry Branch of chemistry concerned with the three dimensional aspects of molecules Three dimensional structures determine properties and biological behavior of molecules s-bonds are cylindrically symmetrical Cylindrical symmetry permits rotation around carboncarbon bonds in open-chain molecules Rotation occurs around the carbon-carbon single bond in ethane Conformations The three-dimensional shape of a molecule at any given instant, assuming that rotation around single bonds is frozen can be represented in two ways: 1. Sawhorse representation Views carbon-carbon bond from an oblique angle and indicates spatial orientation by showing all C-H bonds Conformations 2. Newman projection Views carbon-carbon bond directly end-on and represents the two carbon atoms by a circle Bonds attached to the front carbon are represented by lines to the center of the circle Bonds attached to the rear carbon are represented by lines to the edge of the circle 6

7 Stability of conformations Perfectly free rotation is not observed in ethane Some conformations are more stable than others Newman projections Staggered Lowest energy, most stable conformation All six C-H bonds are as far away from one another as possible Eclipsed Highest energy, least stable conformation The six C-H bonds are as close to one another as possible Torsional strain The strain in a molecule caused by interaction between C-H bonding orbitals on one carbon with antibonding orbitals on the adjacent carbon Also known as eclipsing strain Accounts for the extra 12 kj/mol of energy present in the eclipsed conformation of ethane Energy minima occur at staggered conformations Energy maxima occur at eclipsed conformations 7

8 3.7 Propane Torsional barrier is 14 kj/mol (3.4 kcal/mol) Eclipsed conformation has three interactions Two ethane-type hydrogen-hydrogen interactions One additional hydrogen-methyl interaction Butane The lowest-energy conformation is the anti conformation The geometric arrangement around a carbon-carbon single bond in which the two largest substituents are 180º apart as viewed in a Newman projection Conformation in which the two methyl groups of butane are as far apart as possible Rotation around C2-C3 bond results in the eclipsed conformation where there are two CH 3 H interactions and one H H interaction Continued bond rotation leads to an energy minimum known as a Gauche conformation The conformation of butane in which the two methyl groups are staggered 60º apart as viewed in a Newman projection Higher energy than the anti conformation even though it has no eclipsing interactions This conformation has 3.8 kj/mol steric strain 8

9 Steric strain The repulsive interaction in a molecule when two groups are closer together than their atomic radii allow Rotation around C2-C3 bond results in the eclipsed conformation where there are two H H interactions and one CH 3 CH 3 interaction This eclipsed conformation has the highest energy Both torsional strain and steric strain are present A total of 19 kj/mol After 0º, the rotation becomes a mirror image of the gauche and eclipsed conformations already seen Plot of potential energy versus rotation for the C2-C3 bond in butane 9

10 The most stable alkane conformation One in which all substituents are staggered The carbon-carbon bonds are arranged anti to one another Decane At room temperature rotation around s-bonds occur so rapidly that all conformations are in equilibrium. At any given time, a larger proportion of molecules will be present in a more stable conformation than in a less stable one Worked Example 3.4 Drawing Newman Projections Sighting along the C1-C2 bond of 1-chloropropane, draw Newman projections of the most stable and least stable conformations. 10